In the NB-WDM system, a main optical signal and a supervisory optical signal have different signal wavelengths, and are propaged through the same optical fiber. In order to prevent the deterioration of the transmission characteristic of the main signal in a long distance transmission, it is very important that in-line amplifier stations distributed along the optical transmission line are controlled based on a correct information on the number of channels (wavelengths) of the optical signals. Accordingly, the information on the number of the wavelengths (the wavelength number information, hereinafter) of the optical signals is established on the format of the supervisory signal (the SV signal, hereinafter) and transmitted to the respective in-line amplifier stations.
However, if a fault arises in a terminal station or the in-line amplifier station, it is indicates that the wrong wavelength number information is supplied to the down-stream in-line amplifier stations. Accordingly, it is extremely desirable that the NB-WDM system is provided with the function of discriminating whether the wavelength number information is reliable or not.